If I were to guess, I'd say the patch has a little more grip and a little more rolling resistance.

I'm about .5 seconds faster without the patch. Without the patch, it sounds like I'm at a higher rpm at the end of the straights. I'm more likely to lose the rear of the car at Griffin's Bend (T2).

With the patch, I'm more consistent. Braking is easier. Braking is much easier at Murray's Corner (the last turn). I don't think I encountered the brake bug once using the patch. Without it, it's easy to lock the left front wheel at Forrest's Elbow (the last sharp turn before Conrod Straight) and Murray's Corner. With the patch I can brake late and still recover with a little loss in time. Without it, many of my late braking mistakes result in a crash or spin. I'm more likely to over heat the rear tires without the patch.

In The Cutting, I'm faster without the patch. I only need to gear down to 2nd and I can use the throttle to point the nose in for the sharpest bend. When I'm using the patch, the rear of the car doesn't slide as much and I need to gear down to 1st. In 2nd gear with the patch, the car wants to understeer into the banking.

At this point, I'd be faster in qualifying without the patch and probably faster in a race with the patch.

At some point, I'll try fine tuning my setup with the patch and without it. I noticed differences in the tire's temperatures. It's not just the corrected values from the patch but differences in temperatures across the tire, inside vs. outside temps. With the patch, my setup might be improved with some minor camber changes and shorter gearing. That might eliminate the differences in lap times.

This patch seems beneficial with the GTs. I'm not inducing the brake bug. I'm not sure it's eliminated it but it's a definite improvement.

I could easily get obsessed with the Lola at Bathurst, so I'll be testing this some more.

I've been playing around with the patch ever since it was released with the can-am 66 mod and I'll second JMF's findings. The handling tends to be more consistent over time but my absolute fastest lap times are unchanged. The mod this will have the biggest impact on is the GT 67's just because of the big difference in optimal tire pressure (32psi) and the bug in the physics (24psi). Look at your tire middle temps before and after the patch to see the effect.

I'm wondering now if most of the differences in handling between the original GPL cars and those of the 1967x mod are not due to this patch applied by default to the latter. The differences you describe are rather similar to the ones I found between the different 1967 F1 cars.